NTSB Identification: CHI05LA243.
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Accident occurred Sunday, August 28, 2005 in El Dara, IL
Probable Cause Approval Date: 05/30/2006
Aircraft: Allensworth Baby Belle, registration: N9094P
Injuries: 1 Fatal.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The experimental amatueur-built helicopter collided with trees and terrain following an in-flight separation of a tail rotor blade. Witnesses reported seeing the helicopter spinning prior to it contacting the terrain. The helicopter was involved in a hard landing in 2004 after which the main rotor blades and tail rotor assembly including the spindle and blades were replaced. According to the pilot's friend, they balanced the new tail rotor blades after they were installed, but the blades continued to run "rough." The pilot voiced these concerns to the manufacturer and the manufacturer sent replacement tail rotor blades to the pilot. According to the manufacturer and the pilot's friend, the helicopter was running fine after the replacement blades were installed. Examination of the wreckage revealed all major components of the helicopter were located at the main wreckage site except for one of the tail rotor blades and the respective outboard 2 inches of the tail rotor spindle which were not located. The tail rotor spindle contained two integral rods, which extended out in opposite directions. The length of each rod was manufactured with sections of consecutively reduced diameter. A metallurgical examination of the spindle revealed a fatigue failure was present, which initiated at the fillet radius between the sections where the rod steps down from 0.812-inch diameter to 0.750-inch diameter. The relief radius in this area was rough and irregular. The manufacturer provided the engineering drawings for the tail rotor spindle. Drawing #15 did not specify a relief radius for the transition area between the .812-inch and .750-inch diameter sections. On September 14, 2005, Canadian Home Rotors (CHR), provided a revised drawing #15 requiring a 0.0156-inch relief radius between the 0.812-inch and 0.750-inch diameter sections. The relief radius measured at the failure was approximately 0.0073-inch.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The manufacturer failed to specify a relief radius on the tail rotor spindle drawings, which resulted in the spindle being manufactured with an inadequate relief radius at the point of fatigue failure.

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